This invention relates generally to noise reduction circuit and, more particularly to noise reduction circuit for video signals having suitable nonlinear processing character.
By convention, the noise reduction circuit in use with VTR (video tape recorder) employs the subtraction processing of the video signal to remove the noise component contained in the video signal. Specifically, the delayed video signal, which is delayed by one frame time (1/30 sec.) behind the input video signal, is reduced from the video signal, thereby to obtain the difference signal as the result of the subtraction. According to the level of the difference signal, the noise is extracted, i.e., the nonlinear processing is executed. The extracted noise is subtracted from the video signal, to reduce the actual noise component in the video signal.
In the nonlinear processing as mentioned above, the characteristics in the nonlinear processing is modified according to the S/N (signal-to-noise) ratio of the video signal. At the same time, the threshold of the nonlinear processing circuit is modified according to the amplitude level, for example. This improves the S/N to increase the noise removal rate, and minimizes the mixing of the after image in the motion picture portion. The motion picture portion indicates the portion of the picture where the video signal of one frame is different from that of another frame. After this noise reduction process, no noise component, no after-image appears on the reproduced picture. An example of the system for reducing the noise in the video signal is described in "Television Gakkai", Vol., 33, No. 4, "Noise Reducer", 1977.
The noise reducing system described in this paper requires complicated circuitry and algorithm. This implies that the cost to manufacture the system is high.